The present invention generally relates to an idle running control apparatus for an internal combustion engine such as of a motor vehicle or car. More particularly, the invention is concerned with a method and an apparatus for protecting the internal combustion engine against the occurrence of misfiring to suppress a resultant hunting phenomenon and an engine stall during idle running operation to thereby ensure a stable operation of the engine.
In an idle running control apparatus for an internal combustion engine known heretofore, the engine rotational number or speed (rpm) is successively measured and feedback control of the engine speed is performed with a predetermined constant gain so that a desired engine rotational speed can be attained while correcting the control gain based on the atmospheric pressure, as disclosed, for example, in Japanese Patent Application Laid-Open No. 131841/1982 (JP-A-57-131841).
However, the known idle running control apparatus is not provided with any measure to cope with the occurrence of misfiring. Consequently, when misfiring occurs in an engine cylinder, a so-called hunting phenomenon takes place in the engine operation notwithstanding the feedback control as mentioned above, giving rise to a problem that the comfortableness in driving a motor vehicle is significantly detracted. In this conjunction, it is further noted that when the desired rotational speed for the normal idle operation is set as low as permissible in order to evade or alleviate hunting or significant fluctuations in the engine speed due to misfiring, there may arise an unwanted situation that the engine is stalled upon the occurrence of misfiring if the predetermined feedback control continues to be effected.